Integrated circuits, such as microprocessors and memory devices, include many metal-oxide-semiconductor field-effect transistors (MOSFETs), which provide the basic switching functions to implement logic gates, data storage, power switching, and the like. Power MOSFETs have typically been developed for applications requiring power switching and power amplification.
In a power MOSFET, it is desirable to have a high breakdown voltage (BV). The breakdown voltage of a device indicates the ability of the device to withstand breakdown under reverse voltage conditions. In a typical transistor, much of the breakdown voltage is supported by a drift region. In order to reduce the conductivity of the drift region and provide a higher breakdown voltage, it needs to increase the thickness of the drift region or decrease the impurity concentration of the drift region.
It is also desirable for a power MOSFET to reduce the resistance of the device during conduction (i.e., on-resistance Rds-on). The on-resistance Rds-on is determined by the resistance of the channel and the resistance of the drift region. Specifically, the resistivity of the drift region is determined by the impurity concentration and the thickness of the drift region. In other words, while conductivity of the drift region can be reduced to improve the breakdown voltage, the on-resistance Rds-on would be adversely affected. Accordingly, these exists a trade-off relationship between optimization of on-resistance and breakdown voltage in a conventional transistor.
A superjunction structure provides a way to decrease on-resistance Rds-on of a power MOSFET device without adversely affecting its breakdown voltage. Specifically, it includes alternating P-type and N-type doped columns formed in the drift region. If a reverse bias is applied to the gate structure, the device enters an off-state where a depletion region can be spread at P-N junction between the columns. Since the alternating P and N type columns are in substantial charge balance, these columns deplete one another and allow the device to sustain a high breakdown voltage. For a superjunction structure, the on-resistance Rds-on increases in direct proportion to the breakdown voltage BV, which is a much less dramatic increase than in the conventional semiconductor structure. A superjunction structure may therefore have significantly lower Rds-on than a conventional MOSFET device for the same high breakdown voltage (BV) (or conversely may have a significantly higher BV than a conventional MOSFET for a given Rds-on).
In addition, the significant amount of current through the device can lead to significant electric field (E), which can damage the device absent sufficient safeguards. In order to mitigate the risk of damage due to large electric fields, a termination region is disposed at an outer periphery of the active cell region to attenuate electric field and thus preventing breakdown of the device.
It is within this context that embodiments of the present invention arise.